As loupes for medical care, manual work, and the like, there are used spectacle-type loupes to be worn like a pair of glasses, or head-mounting type loupes to be fixed to the head, which allow manual work to be performed with both hands. Such a loupe is expected to be reduced in size and weight to reduce the burden of mounting, with various aberrations corrected from the center to the periphery of the loupe view so as not to disturb manual work.
Patent Literature 1 discloses an observation system for use as binoculars or the like, the observation system including an eyepiece lens that provides a wide angle of view while downsizing the entire system. However, the optical system of Patent Literature 1 is configured to have positive and negative objective lenses, and therefore reducing the size and weight for ease of wearing results in insufficient correction of aberration and fails to ensure good performance.